ROEVER COLLEGE OF ENGINEERING & TECHNOLOGY
ELAMBALUR, PERAMBALUR-621220
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
DESIGN OF ELECTRICAL MACHINES
UNIT I INTRODUCTION
Part – A- Two marks questions
1. Define specific magnetic loading.
2. Define specific electric loading.
3. What is optimum design?
4. Define rating.
5. Mention the different types of duties.
6. State the properties of insulating materials.
7. What are the major considerations to evolve a good design of electrical machines?
8. What are the major limitations in machine design?
9. How materials are classified according to their degree of magnetism?
10. Classify the insulating materials.
11. Write the requirements of conducting materials.
12. Define duty factor.
13. List the types of magnetic materials.
14. Write down the insulating materials used for wires.
15. What do you mean by standard specifications
Part – B 16 marks questions
16. Discuss about the various types of thermal ratings of the electrical machines
17. Discuss about the various Insulating materials and their grades.
18. Discuss about major considerations and limitations in machine design.
19. What are the main groups of electrical conducting materials? Describe the properties and
applications of those materials.
20. Derive the expression for heating and cooling co efficient.
21. A field coil has a heat dissipating surface of 0.15m2 and a length of mean turn of 1m. It
dissipates loss of 150w, the emissivity of 34 W/m2 .Esstimate the final steady state
temperature rise of coil and its time constant if the area of coil is 100X50 m2, Specific heat
of copper is 390 J/Kg-c .The space factor is 0.56.Copper wewighs 8900kg/m3
UNIT II DC MACHINES
Part – A- Two marks questions
1. What is real and apparent flux density?
2. Define field form factor.
3. Define i)gap contraction factor for slots. ii)gap contraction factor for ducts.
4. Define stacking factor.
5. Define leakage coefficient.
6. List the guiding factors for selection of number of poles.
7. Define commutator segment pitch.
8. Name any two methods to reduce armature reaction.
9. What is slot loading?
10. List the methods used for estimating the mmf for teeth.
11. What are the different types of leakage fluxes?
12. What is unbalanced magnetic pull?
Part – B 16 marks questions
1. Determine the air gap length of a D.C. machine from the following particulars:
Gross length of core = 0.12 m; number of ducts = one and is 10mm wide. Slot pitch = 25
mm; slot width = 10mm; Carter’s coefficient for slots and ducts = 0.32, gap density at pole
centre =0.7Wb/m2, mmf for the magnetic circuit = 3900A; mmf required for iron parts of
magnetic circuit = 800A.
2. calculate the mmf required for the air gap length of a machine having core length of 0.32m
including 4 ducts of 10mm each pole arc=0.19m, slot pitch=65.4mm ,lg=5m, flux=52
mwb,Kcs=0.18 for opening /gap=1 & duct is 0.28 opening/gap=2
3. 3.A laminated tooth of armature steel in an electrical machine is 30 mm long and has a tapper
B(wb/m^2)
1.6
1.8
1.9
2.0
2.1
2.2
2.3
at
(x10^3)A/m
3.7
10
17
27
41
70
109
such that maximum width is 1.4 times of minimum width. Estimate the mmf required for a
mean flux density of 1.9 wb/m2 using Simpson's method.
4. Explain in detail about the selection of number of poles.
5. Derive an expression of output equation for dc machine.
6. Complete the following design for a 92 kW, 220 V, 1480 rpm, d.c shunt motor having full
load efficiency of 89.76%. Assume a specific magnetic & electric loading (Bav) as 0.545T
and 32750 ac/m. Given that the ratio pole arc/pole pitch = 0.67. Select a design which gives a
square pole face.
i. Number of poles
ii.
iii.
iv.
v.
Length of core
Diameter of armature core
Pole pitch
Armature mmf per pole
7. Derive the relation between real and apparent flux densities (Breal & Bapp.).
8. Find the main dimension of 200 kW, 250 V, 6 pole, 1000 rpm generator. The maximum value
of flux density in the gap is 0.87 Wb/m2 and the ampere conductors per meter of armature
periphery are 31,000. The ratio of pole arc to pole pitch is 0.67 and the efficiency is 91%.
Assume the ratio of length of core to pole pitch = 0.75.
9. Determine the diameter and length of armature core of a 55 kW, 110 V, 1000 rpm, 4 pole
shunt generator, assuming the specific electric and magnetic loadings as 26000 amp.cond/m
and 0.5Wb/m2 respectively. The pole arc should be about 70% of pole pitch and length of
core about 1.1 times the pole arc. Allow 10 ampere for the field current and assume a voltage
drop of 4 volt for the armature circuit. Specify the winding used and also determine suitable
value for the number of armature conductors and the number of slots.
10. Compute apparent flux density Bapp in teeth of dc machine when the real flux density is 2.15
wb/m2,slot pitch is 28 mm, slot width is 10 mm and gross core length is 0.35m, the no. of
ventilating ducts 4 each of 10 mm, mmf and a flux density is 2.15 wb/m2 & 55000 A/m,
Ki=0.9 .
11. Design a suitable commutator for a 350kW, 600 rpm. 440V, 6 pole d.c. generator having an
armature diameter of 0.75m, the number of coils = 288. Assume suitable values wherever
necessary.
UNIT III TRANSFORMERS
Part – A- Two marks questions
1. Write down the output equations of single and thee phase transformer?
2. Define window space factor.
3. why stepped cores are used for transformer?
4. Distinguish between core and shell type transformer.
5. Give the relation between emf per turn and kva rating of machines
6. What are the factors affecting the choice of flux density of core in a transformer.
7. List the different methods of cooling of transformer.
8. How heat dissipates in transformer?
9. Define iron space factor.
10. Write down the equation for leakage reactance.
Part – B 16 marks questions
1. Deduce the expression for output equation of single phase and three phase transformer.(16)
2. Calculate the core and window areas required for a 1000 kVA, 6600/400 V, 50Hz, single
phase core type transformer. Assume a maximum flux density of 1.25 Wb/m2 and a current
density of 2.5 A/mm2. Volt per turn = 30 V and window space factor = 0.32.
3. Determine the dimensions for core and yoke for a 5 kVA, 50 Hz, single phase core type
transformer. A rectangular core is used with long side twice as long as short side. The
window height is 3 times the width, voltage per turn is 1.8 V, space factor 0.2; current density
1.8 A/mm2, flux density 1 Wb/m2.
4. Estimate the main dimension including winding conductor area of a 3 phase. ∆/λ core type
transformer rated at 300 kVA, 6600/440 V, 50 Hz. A suitable core with three steps having a
circumscribing circle of 0.25m diameter and a leg spacing of 0.4m is available. The emf per
turn is 8.5 V. Assume a current density of 2.5 A/mm2, a window space factor of 0.28, and a
stacking factor of 0.9.
5. Determine the main dimensions of the core, the number of turns, the cross sectional area of
conductors in primary and secondary windings of a 100 kVA, 2200 / 480 V, 1- phase, core
type transformer, to operate at a frequency of 50 Hz, by assuming the following data.
Approximate volt per turn = 7.5 volt. Maximum flux density = 1.2 Wb / m2. ratio of effective
cross – sectional area of core to square of diameter of circumscribing circle is 0.6. Ratio of
height to width of window is 2. Window space factor = 0.28. Current density = 2.5 A/mm2.
6. Calculate the dimensions of the core, the number of turns and the cross sections of the
conductors for a 100 kVA, 2300/400 V, 50 Hz, single phase Shell Type Transformer
assuming: ratio of magnetic and electric loadings (i.e., flux and secondary mmf at full load),
480×10–8, maximum flux density 1.1 Wb/m2, current density 2.2×106 A/m2. Window space
factor, 0.3, ratio of depth of stacked core to width of central limb, 2.6, ratio of height to width
of window, 2.5; stacking factor 0.9.
7. The tank of 1250 kVA, natural oil cooled transformer has the dimensions length, width and
height as 0.65 x 1.55 x 1.85 m respectively. The full load loss = 13.1 kW, loss dissipation due
to radiations = 6 W / m2-°C, loss dissipation due to convection = 6.5 W / m2°C,
improvement in convection due to provision of tubes = 40%, temperature rise = 40°C, length
of each tube = 1m, diameter of tube = 50mm. Find the number of tubes for this transformer.
Neglect the top and bottom surface of the tank as regards the cooling.(16)
8. Describe about the optimum design of transformer
9. Explain in detail about cooling methods or transformers
UNIT IV INDUCTION MOTORS
Part – A- Two marks questions
1.
2.
3.
4.
5.
6.
Define dispersion coefficient.
What is crawling and cogging?
What is run away period?
List the main parts of an induction motor
Why length of airgap in an induction motor kept at minimum possible range?
What are the factors to be considered for estimating the length of aigap in an induction
motor?
7. How crawling can be prevented by design in an induction motor?
8. Write-down the output equation of an induction motor?
9. Define stator slot pitch.
10. What are the factors to be considered for the choice of specific electric loading?
11. Why wound rotor construction is adopted?
12. How the induction motor can be designed for best power factor?
13. Where mush windings are used?
14. What are the methods adopted to reduce harmonic torques?
15. What happens if the air-gap of an induction motor is doubled?
Part – B 16 marks questions
1. 1.Deive the expression for output equation of three phase induction motor.(8)
2. Determine the approximate diameter and length of the stator core, the number of stator slots
and the number of conductors for a 11 kW, 400 V, 3φ, 4 pole, 1425 rpm delta connected
Induction motor. Adopt a specific magnetic loading of 0.45 Wb/m2 and a specific electric
loading of 23,000 A/m. Assume full load efficiency and power factor as 0.85 and 0.88
respectively. The ratio of core length to pole pitch is 1. The stator employs a double layer
winding.
3. Find the values of diameter and length of stator core of a 7.5 kW, 220V, 50 Hz,4 pole, 3
phase induction motor for best power factor. Given: Specific magnetic loading = 0.4 Wb/m2;
specific electric loading = 22000 A/m; efficiency = 0.86 and power factor = 0.87. Also find
the main dimensions if the ratio of core length to pole pitch is unity.
4. Calculate the equivalent resistance of rotor per phase with respect to stator, the current in
each bar and end ring and the total copper loss for a 415 V, 50 Hz, 4 pole, 3 phase induction
motor having the following data:
Stator:
Slots = 48, conductors in each slot = 35, current in each conductor = 10 A
Rotor: Slots = 57; length of each bar = 0.12m area of each bar = 9.5 5.5 mm2 and mean
diameter of end ring = 0.2 m area of each end ring = 175 mm2. Resistivity of copper is
0.02 /m and mm2.Full load power factor is 0.85.
5. Design a cage rotor for a 18.8Hp, 3phase, 440V, 50Hz, 1000 rpm, induction motor,having
full load efficiency of 0.86, P.F = 0.86. D= 0.25m, L=0.14m, Zss/Ss = 54.Assume missing
data if any.
6. A 15 kW, three phase, 6 pole, 50 Hz, squirrel cage induction motor has the following data,
stator bore dia = 0.32m, axial length of stator core = 0.125 m, number of stator slots = 54,
number of conductor / stator slot = 24, current in each stator conductor = 17.5 A, full load P.F
= 0.85 lag. Design a suitable cage rotor giving number of rotor slots section of each bar and
section of each ring. The full speed is to be 950 rpm, use copper for rotor bar and end ring
onductor. Resistivity of copper is 0.02ohm-m.
7. A 90 kW, 500V, 50 Hz, three phase, 8 pole induction motor has a star connected stator
winding accommodated is 63 slots with a 6 conductors / slot. If slip ring voltage, an open
circuit is to be about 400V at no load find suitable rotor winding. Calculate number of rotor
slots, number conductors / slot, coil span, number of slots per pole. P.F = 0.9 and the
efficiency is 0.85
8. Determine the approximate diameter and length of stator core, the number of stator slots and
the number of conductors for a 20 kW, 400V, 3 phase, 4pole, 1200rpm, delta connected
induction motor. Bav =0.5T, = 0.82, ac = 26,000 amp.cond/m, power factor = 0.8, L/ = 1,
double layer stator winding.
UNIT V SYNCHRONOUS MACHINES
Part – A- Two marks questions
1.
2.
3.
4.
5.
Define short circuit ratio?
What is run away speed?
Write down the factors to be considered for the selection of armature slots.
Give the need of damper winding in synchronous machine.
Write down the equation for no load field mmf per pole and also the length of air gap
at the Centre of the pole.
6. Differentiate between salient pole and cylindrical pole alternators.
7. Mention the factors that govern the design of field system of the alternator.
8. What is skewing?
9. What are the different methods for the elimination of harmonics from the generated
voltage?
10. What are the effects of SCR on machine performance?
11. What is approximately the runaway speed of Kaplan turbine?
12. What are the disadvantages of designing the alternators with higher gap flux density?
13. What is critical speed of alternator?
14. Why salient pole construction is rejected for high speed alternators.
15. What material user for the construction of turbo alternator rotor.
Part – B 16 marks questions
1. Determine suitable stator dimensions for a 500 kVA, 50 Hz, 3 phase alternator to run at 375
rpm. Take mean gap density over the pole pitch as 0.55 Wb/m2, the specific electric loading
as 25,000 A/m. The peripheral speed should not exceed 35 m/s.
2. Determine for a 250 kVA, 1100V, 12 pole, 500 rpm, 3φ alternator (i) air gap diameter (ii)
core length (iii) number of stator slots (iv) cross section of stator conductors assuming
average gap density as 0.6 Wb/m2, ampere conductors per metre as 28,500; and the current
density to be 3.5A/mm2.
3. A 3 phase, 6600 V, 50 Hz, star connected alternator is to run at 750 rpm. There are to be 36
conductors per slot, the flux per pole is 42 ×10-3 Wb and the distribution factor is 0.97. The
winding is concentric with overhang arranged in two planes. Calculate the number of slots
4.
5.
6.
7.
8.
required
A 500 MW, 3 phase, 2 pole, 50 Hz direct water cooled generator has a diameter of 1.3mand a
core length of 5.9 m. The number of turns per phase is 11 and there are two parallel circuits
per phase. The average gap density is 0.575 Wb/m2 and the winding factor is0.92.Determine
the terminal voltage of the machine. Find also the specific electric loading if the power factor
is 0.85.If the mmf required for air gap is 80% of no load mmf, short circuit ratio 0.55, air gap
contraction factor 1.1, determine the length of air gap.
The field coils of salient pole alternator are wound with a single layer winding of bare copper
strip 30mm deep, with separating insulation 0.15 mm thick. Determine a suitable winding
length, number of turns and thickness of conductor to develop an mmf of 12000A with a
potential difference of 5V per coil and with a loss of 1200 W/m2 of total coil surface. The
mean length of turn is 1.2m. The resistivity of copper is 0.21 Ω/m and mm2
A 1250 kVA, 3 phase, 6600 V, salient pole alternator has the following data and air gap
diameter = 1.6 m; length of core = 0.45 m, number of poles = 20, armature ampere
conductors per metre = 28000; ratio pole arc to pole pitch = 0.68; stator slot pitch = 28 mm;
current density in damper bars = 3A/mm2. Design a suitable damper winding for the machine.
.Find main dimension of 100 MVA, 11 kV, 50 Hz, 150 rpm, three phase water wheel
generator. The average gap density = 0.65 wb/m2 and ampere conductors / m are 40000. The
peripheral speed should not exceed 65 m/s at normal running speed in order to limit runaway
peripheral speed.
Determine the main dimensions for a 1000 KVA, 50 Hz, 3 phase, 375 rpm alternator. The
average
gap density is 0.55 wb/m2, ac=28,000 AC/m, use regular poles and assume
suitable values of ratio of core length to pole pitch in order to use bolted on pole construction
for which the maximum permissible peripheral speed is 50 m/s. The runway speed is 1.8
times of synchronous speed.